1. Field of the Invention
The present invention generally relates to hydrostatic transmissions and, more particularly, is concerned with hydrostatic transmission track ring dampeners.
2. Description of the Prior Art
Hydrostatic transmissions transmit rotary mechanical motion, typically from an internal combustion engine, to fluid motion, then back to rotary mechanical motion to rotate a drive axle in order to drive a vehicle such as a lawn and garden tractor or riding mower. The hydrostatic transmission regulates or controls the output rotary mechanical motion such that varying output speeds in the forward and reverse directions are possible with a single speed input rotary mechanical motion. Within a hydrostatic transmission of the radial piston type, a pump and motor each has a cylinder unit that rotates on a fixed pintle with pistons positioned within the cylinders and attached to slippers mounted in an expander band so that as the cylinder unit rotates, the pistons engage a surrounding eccentric annular track ring. The pistons of the pump create a pressurized fluid flow that drives the motor pistons which rotate an output shaft. The transmission ratio is therefore directly proportional to the eccentricity of the track ring of the pump relative to the fixed pintle.
The eccentricity of the pump track ring must therefore be variable and this is accomplished by pivoting the track ring around an axis located at one end of the track ring, the axis generally being a pivot pin. In addition, a control mechanism adapted to swing or pivot the track ring around the pivot axis must also be provided so that an operator can change the eccentricity of the track ring. This type of hydrostatic transmission is shown in U.S. Pat. No. 4,979,583, entitled VARIABLE SPEED TRANSAXLE, issued Dec. 25, 1990, and pending application Ser. No. 07/535,462, entitled VARIABLE SPEED TRANSAXLE, filed Jun. 8, 1990, both of which are specifically incorporated herein by reference.
One of the problems associated with hydrostatic transmissions is the vibration of the pump track ring caused by the rotating pistons/slippers engaging the track ring. The vibration of the track ring creates undesirable noise that is transmitted through the transmission into all of the transmission parts and into the ambient.
One prior art solution to the vibration/noise problem is to position hydraulically actuated pressure pads on opposite lateral sides of the pump track ring to thereby squeeze the track ring between the pads and thus reduce the vibration and associated noise by severely limiting lateral movement of the track ring. The hydraulically actuated pressure pads utilize the hydraulic fluid and pressure within the hydrostatic system for operation.
Although the prior art solution of hydraulically actuated pads effectively reduces the vibration and associated noise, the problem or disadvantage to the prior art solution is that the pressure of the pads is being exerted by means of hydraulic force. The force that is exerted by the pads is directly proportional to the pressure of the hydraulic fluid in the system, while the pressure of the hydraulic fluid in the system is dependent upon the amount of torque that is being exerted on the output shaft. Therefore, the pads exert a force, or squeezing action, against the pump track ring that is dependent upon the torque exerted upon the output shaft. If the torque high, the pads will squeeze against the track ring tightly, if the torque is low, the pads will squeeze against the track ring lightly.
This torque dependency presents a problem in many situations. Hydrostatic transmissions are generally used on riding mowers and tractors that employ manual, clutch transmissions. If the clutch is quickly released with the control lever moved into the forward or reverse position, the high instantaneous torque exerted on the output shaft will cause momentarily high hydraulic fluid pressure, subsequently causing the prior art hydraulically actuated pads to tightly grip the track ring and resist the ability of the operator to move the control back into the neutral position. If the operator releases the control lever to a neutral position, it will encounter resistance since the high torque will produce a high fluid pressure which will cause the track ring to be gripped too tightly and thus resist any movement at all.
A further disadvantage of the prior art relates to the desirability for the operator to have a "feel" of the control lever to indicate when the mower or machine is operating under high torque conditions so that the amount of applied power may be adjusted accordingly. In a hydraulic transmission not employing a vibration damper, if the operator is encountering an incline while pulling a heavy load, for example, the increased torque exerted on the output shaft will be translated into increased oil pressure, which will provide pressure on the control lever tending to move the track ring and control lever back to the neutral position. The operator can sense this feedback pressure and adjust the speed of the mower or machine accordingly by moving the control lever towards the neutral position, thereby reducing the high torque conditions. In the prior art hydraulically actuated pads, the track ring will be clamped more tightly due to the high torque produced oil pressure condition and thus will resist the movement towards neutral, whereby the "feel" or feedback pressure will be lost or significantly reduced.